1. Field of the Invention
The present invention relates to a protecting system for transmission lines and, more particularly, to a transmission line protecting system which is enabled to forcibly ground a faulted transmission line by the use of grounding switches thereby to permit rapid auto reclosure.
2. Description of the Prior Art
Most of the faults occuring in a power system, especially in transmission lines, are caused by an arc resulting from a lightning strike. Therefore, the possibility of recovering the power supply is enhanced to a high level by reclosing the breaker of the transmission line under consideration after the breaker has been opened. This reclosure of the breaker is remarkably effective for maintaining the stability of the system. Therefore, the main transmission lines in recent years have been equipped with a reclosing system as an important function of a protecting relay device.
The following systems are adopted at present as examples of such reclosing systems:
(a) Single-Phase Reclosing System:
When a single phase earth fault occurs, only the faulted phase is selectively opened, and the corresponding breaker is reclosed after a predetermined time period (which will be referred to as the "dead time") sufficient for extinguishing the arc.
(b) Three-Phase Reclosing System:
All three phases are broken independently of the faulted phase, and the reclosure is conducted.
(c) Multi-Phase Reclosing System:
This system is applied to a two-circuit parallel transmission line arrangement. Only the faulted phases are selectively opened, and the reclosure is conducted in case the summed healthy phases for the two circuits are two or more different phases (e.g., A and B phases).
Here, the time period between the opening and the reclosure of the breaker, i.e., the dead time is set by the time, period for which the arc of the faulted point disappears. The dead time is selected at about 0.5 secs. for the transmission lines of 275 KV and about 1 sec. for the transmission lines of 500 KV. It is well known in the prior art that the time period for the arc to disappear generally gets longer as:
(a) the voltage becomes higher; and PA0 (b) the transmission lines are made longer.
The reason therefor can be explained, as follows:
If three-phase transmission lines are faulted (e.g., if it is assumed that the a-phase is grounded to the earth) so that the a-phase breakers at both the terminals are opened, the energy supply to the faulted point of the a-phase from the generator (of the system) is eliminated so that the arc ought to disappear. However, since the b and c phases are left energized, the energy is supplied from the healthy phases, b and c to the faulted point through an interphase capacity. In case the voltage is low and the transmission lines are short (i.e., the capacity coupling is low), that energy supply can be ignored so that the arc naturally disappears. As the voltage gets higher and as the line length gets larger, the charging current through the interphase capacity is increased more. On the other hand, the potential to be determined by the capacity ratio to the earth is left in the broken phase to make the arc reluctant to disappear. In the transmission lines of about 1,000 KV scheduled to be constructed in the future, the estimations have reported that the arc is reluctant to disappear and takes several seconds, for example, until it disappears. The arc of this kind, which is induced by the healthy phases, is called the secondary arc.
On the other hand, it is desired for the stability of the system that the dead time be as short as possible. This will be briefly explained in the following. As is well known in the art, the phase difference angle of the voltage between the two terminals of a faulted transmission line is gradually increased until a deceleration energy corresponding to the acceleration energy received by the generator on account of the fault is obtained. As is also well known in the art, if the phase difference angle exceeds 90 degrees, the system becomes unstable and is brought out of control before long so that both the terminals of the transmission line have to be isolated for all the three phases. Moreover, if the breaker is reclosed after lapse of the dead time until the state before the fault can be restored, a higher deceleration energy (than that in the running state during the dead time) can be received. As a result, the possibility of restoring the stable running operation before the phase difference angle becomes large is higher as the dead time becomes shorter. If several seconds or more are required for the dead time, as has been described in the above, however, there arises a serious problem that the stability cannot be maintained.
The adoption of a forcibly grounding system using grounding switches is effective for the means for solving the problems thus far described. According to this system, the potential at the transmission line is forcibly lowered to abruptly extinguish the arc by grounding both the terminals of the opened line after the faulted phase has been opened. This forced grounding system is disclosed, for example, in IEEE Transactions on Power Apparatus and Systems, Vol. PAS-100, No. 4, April 1981, "The Application of High-Speed Grounding Switches for Single-Pole Reclosing on 500 KV Power Systems" R. M. Hasibar, A. C. Legate, J. Brunke, W. G. Peterson, Bonneville Power Administration Portland, Or.
The system disclosed in the above will be explained in the following. First of all, there are connected between power plants A and B three-phase transmission lines of a, b and c phases, each of which has both its terminals connected with breakers CB. Each of these breakers CB has their line-side terminals grounded to the earth through grounding switches ES, respectively. In a normal state, the breakers CB are closed, and the grounding switches ES are opened so that the elimination of a fault and the reclosing operation are conducted by the following procedures in case the fault occurs at a point F of the a phase:
(1) Breakers CB-A and CB-B at both the terminals of the a phase are opened by the actions of the protecting relay devices which are connected with both the terminals of the line.
(2) After the breakers CB-A and CB-B have been opened, grounding switches ES-A and ES-B at both the terminals of the faulted phases are closed to ground at both ends.
(3) After the dead time, the grounding switches ES-A and ES-B are opened.
(4) Next, the breakers CB-A and CB-B are closed.
The secondary arc current can be abruptly reduced by the processing operations according to the foregoing procedures disclosed in the aforementioned paper so that the dead time can be shortened. Thus, the forced grounding system using the grounding switches is effective.
However, it has been revealed that the forced grounding system invites the following disadvantages if the aforementioned processing procedures are mistaken. For example, the grounding switch ES-B is closed before the breaker CB-A is opened, or the breaker CB-A is closed before the grounding switch ES-B is opened. Thus, the operating procedures are liable to be reversed between the devices at the terminals A and B. The former case is equivalent to that the a-phase transmission line being short-circuited at its two different points (i.e., at the arc fault point and the point of the earth switch ES-B) to the earth. No special fault takes place if a breaker having breaking capability is red as the grounding switch. In many cases, however, since an inexpensive switch having no breaking capability is used, there is a fear that the switches may be damaged by the grounded current. On the other hand, the latter case is equivalent to that situation where the grounding fault occurs simultaneously with the reclosure of the breaker CB-A. In this case, the protecting relay device judges that the reclosure has failed to open all the phases a, b and c. Because the transmission lines are major ones intrinsically having a high capacity, the reclosing system is adopted so that the running operation may be devised to continue as long as possible even in the case of a fault. Moreover, the forced grounding system is adopted to improve the stability of the system. Nevertheless, the adoption of that forced grounding system may possibly deteriorate the working efficiency of the power system or the stability of the system.